Virulent Treponema pallidum organisms, extracted from infected rabbit testes, are being subjected to velocity sedimentation in high density, low viscosity materials. Treponemes separate into two distinct bands based upon their relaive velocities and analysis of these treponemal populations is underway. Virulent treponemes are also being monitored for their ability to incorporate amino acids into protein. Methods used to demonstrate that treponemes actively synthesize protein in vitro include use of radioisotopes, radioautography, and selective inhibition of protein synthesis by drugs.